The reproduction of 3D audio has seen significant changes in its delivery to the user. This started with the introduction of multichannel reproduction devices such as the 5.1 loudspeaker systems, which have become only partially popular mainly due to their limited practicality (multiple loudspeakers and cables arranged in the room). Nowadays, the audio consumer market is heading towards the use of more compact solutions such as sound-bars. Evidence of that is provided by the sales figures of these devices, which have increased considerably in the last couple of years. Recently, the home audio market has also seen the introduction of new sound reproduction platforms, such as mobile phones or tablets. Attempts have been made by some manufacturers to produce accessories for these devices to reproduce 3D audio.
Loudspeaker array technology for the reproduction of 3D audio is becoming very attractive, especially because of the decreasing cost of the processing electronics. This allows for the creation of personalized sound zones, in which different users can listen to different audio material without interfering with each other. Additionally, binaural audio reproduced by arrays is likely to become increasingly important in the field of sound reproduction. Binaural audio, initially designed for headphones, is the object of an intense research work carried out by many academic groups, companies, and broadcasters, which are currently developing new solutions and investing in this technology. The reproduction of this audio material with loudspeaker arrays brings the reproduction of 3D audio to another dimension, allowing high audio realism to the consumer.
A number of solutions and proposed ideas for the reproduction of binaural audio through loudspeakers (sometimes also referred to as Transaural audio) are available, as referenced in more detail below. All these systems rely on the use of two or more loudspeakers and of a signal processing apparatus for generating the loudspeaker signals, usually including a network of digital filters to process the input audio signal. Some approaches have been proposed for the adaptive reproduction of binaural audio material, which means that the digital signal processing (DSP) algorithm is adapted depending on the position of the listener(s). These adaptive systems make use of a database of digital filters for a number of predefined listening positions and then select the filters that best match the position of the listener. The drawback of these approaches is that the database of digital filters needs to be pre-calculated and also a carefully tuned signal processing scheme is required to change between the filters associated to different listener positions without compromising the delivered audio quality. Therefore, these systems have a limited operational range, which is given by the size of the grid for which the filters have been created, and their application is limited by the high computational load required for their implementation.
To overcome this limitation in operation range and provide a personal localised; and or binaural reproduction, improved DSP strategies, such as the one disclosed herein, may be implemented.
The concept of a loudspeaker array has existed since the 1940s; however its use for audio applications has not become spread until the 1990s, introducing a paradigm change in PA applications, as much less power was needed to obtain a better distribution of audio over a large audience. In the field of home audio, it has not been until very recently that the use of sound-bars for home cinema applications has become popular. Many of the sound-bars that are now available in the market use traditional array technologies, and although they do provide a higher quality than built-in speakers which are part of many television sets nowadays, their spatial performance is limited.
In order to provide a better spatial audio performance, it is possible to use cross-talk cancellation techniques. A concept firstly introduced by Atal and Schroeder in 1966 [1], cross-talk cancellation for audio reproduction showed itself as an effective idea, however practically limited by the technology available at the time. This was further developed in the 1990s to lead to optimum loudspeaker arrangements as the stereo dipole [2]. In the early 2000s Takeuchi and Nelson presented the concept of the OPSODIS [3], a three way stereo dipole system which ensured to maximise the spatial performance as well as the audio quality.
The use of loudspeaker arrays for cross-talk cancellation has been previously considered by various inventors including Bauck [4], Kuhn et al. [5], Li [6] and Hooley et al. [7], using the same principle as the previously cited patents but with a larger number of loudspeakers.
A drawback of the known cross-talk cancellation reproduction devices however is that they are not adaptive to the position of the listener and constrain the listener to be in the sweet-spot of the sound field. So as to allow the listener to move freely whilst listening to the audio, some systems employ listener tracking, as this for example by Hooley et al. [9]. Another example was presented by Mannerheim et al. [10]. This latter approach works by creating a database of various cross-talk cancellation filters and switching the different (stored and predetermined) filters according to the listener position. Therefore, these filters have to be pre-calculated to account for a large number of potential listener positions, and hence large memory requirements are needed. Apart from this, their performance is constrained by the size of the grid used to calculate the filters and they do not provide an efficient cross-talk cancellation when the listener head is between two grid positions.
We have devised an improved sound reproduction system.